1. Field of the Invention
The invention relates to a reading system for magnetic information comprising at least one read head provided with two magnetoresistive rods whose electrical resistivities vary in opposite senses from one another when they are submitted to a same variation in magnetic field, each magnetoresistive rod having a first and a second connection terminals, the first connection terminal being connected to a negative supply terminal, the second connection terminal, called output terminal of the magnetoresistive rod, being connected via a current source to a positive supply terminal, which read head is in addition provided with a differential amplifier having two inputs each of which is connected to the output terminal of one of the magnetoresistive rods.
2. Description of the Related Art
Such a reading system is known from U.S. Pat. No. 5,420,736. This reading system is, in theory, insensitive to an accidental contact between the read head and a foreign body or asperity which might be present on the surface on which the information is stored. Indeed when such an event takes place, referred to as "thermal asperity" hereinafter, it generates a sudden increase in the resistivity of the magnetoresistive rods. As a result, a sudden increase in voltage will then appear at the output terminal of each magnetoresistive rod. Since the resistivity increases have the same amplitude for the two magnetoresistive rods, the voltage increases also have the same amplitude. The differential amplifier which receives the voltages from the output terminals of the magnetoresistive rods has a high common mode rejection ratio, so that these identical increases of the input voltages of the differential amplifier will be ignored.
It is nevertheless useful to monitor such events. Too violent or repeated contacts between the read head and foreign bodies or asperities present on the surface on which the data are stored are capable of damaging the read head or at least giving rise to reading errors, or also of changing the data stored in the locations where these contacts take place. A control device governing the reading system, if informed of a thermal asperity, may choose to ignore the data read at the locations where this event takes place, or command the read head to skip the location where this event has taken place in future, or alternatively may rearrange the storage of data such that it is no longer necessary to guide the read head over this location. The present invention accordingly has for its object to enable the reading system to detect and signal thermal asperities.